Directional antennas are preferred where the relationship between the sender and receiver are known geographically. In such arrangements, directionality maximizes the power of transmission from sender to receiver.
Omni-directional antennas are a better choice where the location of the recipient is either not known a priori, or in situations where the sender and/or receiver may be mobile. Omni-directional antennas are therefore typically used in local-area network (LAN) and wireless (i.e., wi-fi) environments.
Broadly speaking, an omni-directional antenna radiates power substantially uniformly in all directions. The only three-dimensional omni-directional antenna is the isotropic antenna, a theoretical construct derived from actual radiation patterns and used as a reference for specifying antenna gain and effective radiated power. Practical antennas approach omni-directionality by providing uniform radiation or response only in one reference plane, usually the horizontal plane parallel to the earth's surface. Common omni-directional antennas include the whip antenna, the vertically oriented dipole antenna, the discone antenna and the horizontal loop antenna. While these designs perform adequately in some situations, the need always remains for new configurations for emerging applications.